Move documentation to readthedocs.org

Hello @joemcbride,

I've gone through the graphql-dotnet document but I feel the current document doesn't look good. I think we should move our documentation to https://readthedocs.org/

If you say yes, I would like to work on this.

Start by reviewing the current graphql-dotnet documentation and the 11-comment discussion to determine whether moving it to Read the Docs is agreed. The issue does not name files or tests; done would require an agreed migration plan and documentation hosted on readthedocs.org.

Scritto dal modello di indicizzazione a partire dal testo della issue.